A “stem cell” is a generic name for an undifferentiated type of body cell found in tissues of embryos, fetuses and adults, which has the potential of differentiating into a diverse range of specialized cell types.
Stem cells can be classified depending on their potency: pluripotent stem cells, multipotent stem cells and unipotent stem cells. Pluripotent stem cells have pluripotency to differentiate into any type of cells. Embryonic stem cells and induced pluripotent stem (iPS) cells are representative of pluripotent stem cells. Adult stem cells show multipotency and/or unipotency. Among them are hematopoietic stem cells, mesenchymal stem cells, neural stem cells, etc.
In spite of various attempts to utilize the pluripotent human embryonic stem cells in cell therapeutics, the high likelihood of oncogenesis and immune rejection response still remain and are difficult obstacles to overcome.
In case of iPS cells, which are a type of stem cells purposely regressed from fully differentiated adult cells to their initial stage of embryonic stem cell stage by reprogramming, the risk of immune rejection response could be ruled out because they are autologous cells, but the risk of oncogenesis is still a problem to be solved.
As an alternative to solve these problems, mesenchymal stem cells have been suggested because they exhibit immunomodulatory effects and present no risk of oncogenesis. Mesenchymal stem cells are multipotent stem cells that can differentiate into a variety of cell types, including adipocytes, osteoblasts, chondrocytes, myoblasts, neuroblasts, myocardioblasts, hepatocytes, islet beta cells, vascular cells, etc., and are known to have the function of modulating immune responses.
Mesenchymal stem cells may be isolated from various tissues such as bone marrow, umbilical cord blood, adipose tissue, etc., but may not be sufficiently defined because cell surface markers are somewhat different from one another according to the origin from which the mesenchymal stem cells are derived. Generally, if they can differentiate into osteoblasts, chondrocytes and myoblasts, have a spindle shaped morphology, and express the surface markers CD73(+), CD105(+), CD34(−) and CD45(−), such stem cells are defined as mesenchymal stem cells. In this context, mesenchymal stem cells of different genetic origins and/or backgrounds do not significantly differ from one another under the standard definition, but are significantly different from one another in terms of in vivo activity. Further, when mesenchymal stem cells are used as an exogenous cell therapeutic agent, a limited pool of mesenchymal stem cells does not allow other available options, even in spite of low in vivo activity.
In addition, in order to apply the mesenchymal stem cells to clinical practice, it is essential to obtain a large amount of cells in initial stages and to culture them with passages, considering limited quantity of the mesenchymal stem cells which can be obtained from tissues. However, the mesenchymal stem cells form a very heterogeneous group during passages, which affects proliferation, differentiation, and aging of the cells, rendering the mesenchymal stem cells difficult to be developed as therapeutic agents.
In an effort to overcome these problems, Li J et al., Cell Research, 2008; Majore I et al., Cell Communication and Signaling, 2009; Rataiczak M Z et al., Aging, 2012, etc. disclose a variety of methods for culturing mesenchymal stem cells. However, these methods have several disadvantages: heterogeneous cells are obtained by the methods, making it difficult to obtain necessary number of cells for mass production, and the proliferative capacity of the cells decreases after each passage and aging of the cells progresses rapidly. Further, the equipment used in the above articles was not suitable for good manufacturing process (GMP), making its application to actual manufacturing process difficult. As such, there is a need for a method of extracting homogeneous group of cells and mass proliferating them efficiently and with low cost.